1. Field of the Invention
The present invention relates to a color picture tube apparatus.
2. Description of the Related Art
In order to correct an edge of a displayed image to enhance image quality, a method for modulating a horizontal scanning velocity of electron beams is known. According to this method, in general, a pair of auxiliary coils called beam velocity modulation (BVM) coils are provided at a neck portion of a picture tube so as to be integrated with a deflection yoke and a convergence and purity unit (CPU) (see JP57(1982)-45650Y).
The BVM coils improve a visible state between a dark area and a light area of an image displayed on a screen during a horizontal scanning period as follows. The transition of a lighting state is predicted from a video signal waveform. In a period on a dark side during a transition period of a lighting state, electron beams are accelerated so that they are horizontally scanned at a velocity equal to or higher than an average scanning velocity. On the other hand, in a period on a light side during the transition period of a lighting state, electron beams are decelerated so that they are horizontally scanned at a velocity equal to or lower than the average scanning velocity. Thus, among the areas of the transition of a lighting state on a screen, in an area on a dark side, an excitation time of phosphors is shortened to decrease brightness, and in an area on a light side, an excitation time of phosphors is prolonged to increase brightness. Accordingly, the edge of an image is corrected so as to increase the sharpness of the areas of the transition of lightness and darkness.
In the case where the above-mentioned BVM coils for modulating the horizontal scanning velocity of electron beams are provided so as to be integrated with the CPU, an eddy current is excited in an electron gun made of a metallic conductor due to the magnetic flux generated by the BVM coils, and the metallic conductor generates heat, resulting in a reduction in a velocity modulation effect by the BVM coils.
In order to enhance the sensitivity of the BVM coils, a method has been proposed for mounting a magnetic substance, which focuses and intensifies the magnetic field generated by the BVM coils, in the electron gun (see JP6(1994)-283113A).
However, the above-mentioned method has the following problems: since a magnetic substance for focusing a magnetic field is metal, a new eddy current is likely to be generated; a process for welding a new component is necessary, which increases a cost; and furthermore, sufficient sensitivity cannot be obtained.
Furthermore, it also is considered that, in order to decrease crosstalk between the magnetic field generated by the BVM coils and the electron gun metal to minimize the generation of an eddy current, the BVM coils are placed close to the end of a deflection yoke on the electron gun side. However, in this case, the BVM coils are placed close to a horizontal deflection coil, so that crosstalk between the magnetic field generated by the BVM coils and the horizontal deflection magnetic field is increased to cause new ringing. Therefore, there is a limit for placing the BVM coils close to the deflection yoke, and hence, a sufficient increase in sensitivity cannot be realized.
As one measure, as shown in FIG. 9A, a method has been worked out, for mounting a ring-shaped ferrite core 90 on an outer circumference of the BVM coils so that the ferrite core 90 is integrated with a CPU, thereby intensifying the magnetic flux generated by the BVM coils.
However, the ring-shaped ferrite core 90 simultaneously absorbs a vertical deflection magnetic field 92 generated by the deflection yoke 6, thereby weakening the vertical deflection magnetic field 92 on the side of a neck portion of the deflection yoke 6, as indicated by a broken line 94 in FIG. 9A (in FIG. 9A, a Z-axis indicates a tube axis of a color picture tube). Consequently, preliminary deflection of electron beams is decreased, and as shown in FIG. 9B, rasters 96 in the upper and lower portions of a screen are distorted as indicated by broken lines 98, causing image distortion. In order to reduce such raster distortion, it is preferable to make the ring-shaped ferrite core 90 as thin as possible (decrease the size thereof in a tube axis direction), and place it close to a cathode.
Thus, the configuration of placing the ring-shaped ferrite core 90 together with dipole, quadrupole, and hexapole magnet rings of the CPU has been considered.
In an assembly process of a color picture tube apparatus, the dipole, quadrupole, and hexapole magnet rings of the CPU are fixed so as to keep their positions (rotation positions around a tube axis) after adjusting convergence and purity. As a method for fixing the dipole, quadrupole, and hexapole magnet rings of the CPU, the following methods are known: a tightening fixing method for screwing with a lock ring; and an elastic fixing method for fixing by supplying an elastic compression force with an elastic ring having elasticity. In any of the methods, the dipole, quadrupole, and hexapole magnet rings of the CPU are fixed due to the compression force in the tube axis direction.
The ring-shaped ferrite core 90 has a twist and a wave in a thickness direction (tube axis direction) due to sintering in the course of production. Thus, in the case of fixing such ring-shaped ferrite core 90 together with the dipole, quadrupole, and hexapole magnet rings by the above-mentioned tightening fixing method, the ring-shaped ferrite core 90 will be cracked due to the compression force in the tube axis direction during tightening of a lock ring. Furthermore, even in the case of fixing the ring-shaped ferrite core 90 together with the dipole, quadrupole, and hexapole magnet rings by the above-mentioned elastic fixing method, the ring-shaped ferrite core 90 also will be cracked due to the elastic compression force in the tube axis direction and the external force caused by the vibration during transportation and the like. When the ring-shaped ferrite core 90 is cracked, the effect of intensifying the magnetic field of the BVM coils will be reduced.